As marine seismic exploration is now increasingly deployed in deeper waters in the search for oil and gas, one of the major challenges is the existence of strong sea currents in many of the areas considered to have a high hydrocarbon potential. Deep waters such as those found in the Gulf of Mexico, South America, West, South and East Africa are already major targets for hydrocarbon exploration and will likely continue to be so over the coming years. The existence of these strong sea currents make conventional narrow azimuth surveys (NAZ) very difficult to carry out, and often drives up the cost.
One of the major elements of seismic exploration is the illumination of sub-sea geological structures with sound waves or pulses of known shape and form. Consistency of illumination is required so that reflection coefficients and other petrological characteristics can be determined from the reflected sound energy, and often in such a way as to be repeatable over many years (time-lapse seismic imaging for reservoir monitoring). Consequently there have been significant recent developments in the design and implementation of steerable seismic source and receiver arrays. These systems are designed to counter the effects of varying sea current strength to ensure consistent illumination with the desired narrow azimuth.
Another important factor that need consideration in areas beyond the continental margins and in deeper waters is that geological structures have dips that display a general tendency (but not always) to have the most significant rate of change in a direction perpendicular to the coastlines. Unfortunately, it is also generally true that strong sea currents tend to move parallel to coastlines. This is a problem because modern 3D seismic marine exploration, due to its limited azimuthal and sampling in perpendicular to the sail-line direction (“cross-line”), is generally deployed so as to sample data in the predominantly “dip” direction to ensure optimal imaging of the sub-surface. Hence the long felt need for steerable seismic source and hydrophone receiver arrays to counteract or mitigate the effects of cross-currents.
In many cases, seismic data are also acquired by steering the seismic acquisition system in a direction that substantially corresponds to the sea current direction. The drawbacks of this method are that large engine power is required to tow the seismic source and hydrophone receiver arrays when heading into the sea current. Conversely, when the acquisition system heading is with the sea current then the acquisition system speed through the water must be significantly lowered so ensure that the resultant speed along the sea floor is such that constant illumination is maintained. One of the drawbacks of this lower speed through the water is that the proper steering and control of the seismic source and receiver arrays becomes difficult to accomplish.
In narrow azimuth (NAZ) surveys typically every effort is made to fight sea currents and prevent the deviation of source and receiver azimuths from a very narrow range, as represented in FIGS. 1 & 3.
It is now widely accepted in the industry that the acquisition of multi azimuth, wide azimuth or rich azimuth seismic data (MAZ, WAZ & RAZ) provide greatly enhanced quality of the final subsurface images as compared to the results obtained in narrow azimuth (NAZ) surveys.
Seismic acquisition terminology will be explained in the following.
First, the term Common Mid-Point (CMP) is explained. Common mid points are points in the subsurface defined as the mid points between the various seismic source positions and the hydrophone (or geophone) receiver positions in terms of casting and northing, regardless of the depth of the source or receiver or the dip of the geological structures beneath.
Now, CMP bins is explained. A seismic survey area is typically sub-divided into CMP bins. CMP bins are usually defined as rectangular boxes (approaching square wherever possible) covering the whole area. The size of the CMP bins is dependent on the level of geological resolution required to image correctly all the dipping events, structures and geological faults expected in an area. Bin dimensions are typically in the range of 5 to 200 m and will determine the amount of effort required to image the sub-surface. All individual Common Mid-Points falling within the boundary of a CMP bin are labeled as belonging to that bin for further seismic data processing.
Now, Bin Fold is explained. Fold is defined as the number of common mid-points that accumulate within the boundaries of each CMP bin, and is indicative of the “effort” expended to create the final sub-surface image. The bin fold minimum value is one, 1. Other contributors to “effort” are the signal source strength, the number of receiver groups and the number of hydrophones (or geophones) within the receiver groups.
Now, Shot-Point Spacing (SP spacing) is explained. SP spacing is the distance, in the direction of source progression, between successive points where the seismic energy of the source is released into the water.
Now, Minimum Recording Interval is explained. The Minimum Recording Interval is the minimum time required to record seismic returns from deep within the earth and is usually measured from the time at which the source energy is released.
Now, Minimum Cycle Time is explained. The Minimum Cycle Time is the minimum time required by the energy source equipment to become fully energized and ready for the next release. Accordingly, the Minimum Cycle Time is equal to the shortest possible time interval between the consecutive shots which a seismic source of the seismic survey acquisition system is capable of generating.
In the following, explanations of the terms Maximum Line Spacing, Minimum Line Spacing, and Maximum Shot Point Spacing are provided
The Maximum Line Spacing is the spacing of the sail-lines that are a) substantially perpendicular to the average prevailing sea current and b) determined so that the required bin fold is achieved. Consequently, when considering a vessel towing a seismic source and streamers having respective receiver groups across the direction of the sea current, the maximum line spacing is the line spacing that fulfils the condition fold=1 in all CMP bins. The maximum line spacing, Lmax is given by the equation:Lmax=(Ymax/2)*cosine θaccording to which Lmax equals Ymax divided by two and multiplied by cosine θ, where Ymax is the maximum distance between the source position and the farthest receiver group of the farthest streamer, and θ is the angle (measured clockwise) between the average prevailing sea current direction and the direction of the farthest streamer.
The Minimum Line Spacing is the line spacing that gives largest value of the fold in the CMP bins without permitting the acquisition system to track over the same sail-line twice, and is therefore equal to the dimension of the CMP bin in a direction parallel to the average prevailing sea current direction.
The Maximum Shot Point Spacing, SPmax, is the distance interval that permits all CMP bins covered by a particular sail line to have fold and is given by:SPmax=(N/2)*CMPxaccording to which SPmax equals N divided by two and multiplied by CMPx, where N is the number of seismic receiver cables, and CMPx is the CMP bin dimension in a direction substantially perpendicular to the prevailing sea current direction.
The Minimum Shot Point Spacing is the spacing that permits highest possible value of the fold in the CMP bins without permitting the acquisition system to track over the same point twice, and is therefore defined as being equal to the CMP bin dimension in a direction substantially perpendicular to the prevailing sea current direction.
It is expected that sea currents will maintain a constant speed and direction over a distance of a few kilometers or less, but it is also reasonable to expect that over larger distances, the sea current will not follow a straight line but tend to change direction or meander somewhat, although on a larger scale is considered with respect to its average flow direction and speed.
For the purposes of understanding the method of the invention, we define the points at which the survey design is no longer considered substantially perpendicular to the prevailing sea current to be when either of the following conditions are met:
1. With reference to FIG. 9, when the resultant acquisition system movement vector (M), produced by the sea current vector (C) and the acquisition system heading speed vector (V), is too fast and the maximum SP interval is passed either before the Minimum Cycle Time has been achieved or the Minimum Recording Time for the deeper seismic reflections has not been achieved.
2. With reference to FIG. 10, when the acquisition system speed vector (V) is at a maximum and the acquisition system movement vector (M) is too small, and cannot achieve the minimum SP interval within a time period that is equal to 3 times the Minimum Recording Interval.